Generally, for a calculation between different physical quantities including units, the calculation is performed based on string processing.
However, for a calculation between different physical quantities including units and prefixes, if the calculation is performed based on the string processing, the handling may be very complicated because the differences in order of arithmetic expressions and in notation (font and the like) of units and prefixes should be taken into consideration for the string processing.
Moreover, it is difficult to accurately collate the units resulting from the calculation and thus it is difficult to obtain the calculation result without an error.